We have made in situ optical microscope observation for the microstructure control driven by magnetic field in Fe-31.2Pd (at%), CoO and Nd0.5Sr0.5MnO3. These materials exhibit structural transitions, and their low-temperature phases are composed of several crystallographic domains (variants), which are separated by twinning planes. In the case of ferromagnetic Fe-31.2Pd and antiferromagnetic CoO, the magnetic field promotes the twinning plane movement. This movement gives a large strain of several percent and is essentially explained by the fact that the magnetic shear stress, which corresponds to the magnetic anisotropy energy divided by the twinning shear, is larger than the twinning stress. In the case of Nd0.5Sr0.5MnO3, the twinned microstructure of the charge-ordered phase disappears under a magnetic field in association with the melting of the charge-ordered phase. (C) 2008 Elsevier B.V. All rights reserved.